Here are some tips on how to give (Math) Talks. Talks on other scientific topics should be similar.
Source 1: http://www.math.wisc.edu/~ellenber/mntcg/TalkTipSheet.pdf
One tip I found very useful is this: For long talks (1 hour) and above, it is better to use the whiteboard / blackboard. As this will give you “the flexibility to add or omit material as you see fit, and it forces you not to go to fast.”
For shorter talks, it is better to use slides. “A good rule of thumb: you should allow between 30 seconds and 1 minute per slide.” So if you are preparing for a 30 minute talk, around 30-60 slides would be ideal.
Source 2: https://faculty.washington.edu/heagerty/Courses/b572/public/HalmosHowToTalk.pdf
This is advice by the legendary Paul Halmos. His first advice is to “Make it simple, and you won’t go wrong.”
Mathtuition88 
Yes, Paul Halmos autobiography is extremely useful for an a student of Math. In particular, he also mentions that Math can’t be learn in a “step-wise, linear manner”. One has to refer to many books; while reading a math book, if something is found difficult, to skip it, or to leave that book altogether etc. etc.
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His advice about skipping is a wise one. Without skipping one can potentially get stuck there forever. Coming back to the difficult part at a later time is a wiser choice.
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